1. Technical Field of the Invention
The present invention relates to a system for controlling the speed of a Voice Coil Motor (VCM) in order to perform a low-noise ramp loading of Hard Disk heads.
2. Description of Related Art
As is well known in this specific technical field, a Voice Coil Motor (VCM) is a special type of motor operative to move a read/write head in small steps across a magnetic disk, the extent of the movements being controlled by the electromotive force of a coil. The initials VCM will be used herein for brevity to denote a Voice Coil Motor.
It is common to provide the drive for a power stage controlling the speed of a VCM in a discontinuous pulse width modulation (PWM) mode. Pulse Width Modulation will be designated PWM herein for brevity. This operation mode is used by reason of it being simple and demanding no calibration.
The state of the art provides a number of solutions to the problem of feedback driving motors as above by PWM signals. Relevant references are, for instance, Patents Nos. U.S. Pat. No. 6,542,324 B1, EP 1,118,994 A2, U.S. Pat. No. 6,320,717 B1, and U.S. Pat. No. 6,363,214 B1, the disclosures of each which being incorporated herein by reference.
The above-mentioned and other patent references provide the same circuit type, to be generally discussed below with reference to FIGS. 1, 2 and 3a. 
Shown in FIG. 1 is a block diagram of a speed control system for a VCM motor 1.
The system of FIG. 1 includes a discontinuous PWM signal generator 2 operative to generate an enable signal INONOFF to a power stage 3; this stage includes two power transistors TR1 and TR2 having gains K and −K, respectively, and being both supplied the same power supply signal INFIL.
The stage 3 outputs drive signals Out_M and Out_P to the winding of the VCM motor 1, through which motor a current IVCM flows. A current sensor, for example, in the form of a sensing resistor Rsense is connected in series with the winding of the motor 1.
The difference in value between the two output signals Out_M and Out_P sets the value of the electromotive force Bemf issued from the stage 3, when the output stage is in the tristate condition and there is no current. Both signals Out_M and Out_P are taken to the inverting (−) and non-inverting (+) terminals, respectively, of an operational amplifier 6, the latter supplying a signal OUTRAM to power a Sample and Hold sampling block 4. The block 4 receives a synchronization signal INSH from the generator 2 for synchronizing the reading of the electromotive force Bemf from the VCM motor 1 with the frequency of the PWM signal generated by the generator 2; it comprises basically a switch SW which is driven by the signal INSH, and an output capacitor C1. The block 4 is connected to an adder node 7, to which it supplies a signal OUTSH; the value of OUTSH is compared, by means of the adder node 7, to a reference electromotive force value Bemf_ref, thereby to produce an input signal IN+ to a filtering block 5. The filtering block 5 basically comprises a feedback amplifier 8 in an inverting configuration, which amplifier is input a reference voltage value Vref through its non-inverting (+) terminal, and the signal In+ issued from the adder node 7 through its inverting (−) terminal; the filtering block 5 filters and amplifies the signal IN+and supplies the power stage 3.
It can be appreciated from the above example that, during the operations for disk loading or ramp loading the heads, the speed of the VCM motor 1 is controlled by a direct reading of the electromotive force Bemf generated by the movement of the VCM motor 1 itself. In fact, during a time period when the power stage 3 is enabled (in the Current or in the Voltage mode), a current IVCM flows throughout the VCM motor 1 which will generate a suitable torque to keep the speed under control, whereas during a subsequent time period, the power stage 3 will be cut off by a high impedance Rsense (tristate condition) to allow the current flowing throughout the VCM motor 1 to be discharged, and upon completion of the discharging step, the generated electromotive force Bemf to be detected.
The differential amplifier 6 connected to the two outputs Out_M and Out_P then reads the electromotive force Bemf representing the feedback signal of the speed control cycle of the VCM motor 1.
Unfortunately, a side effect of this driving mode is that acoustical noise is generated during the aforesaid ramp loading operations, due to the low working frequency of the discontinuous PWM drive.
In addition, the intensity of this noise can vary according to the frequency of the PWM signal generated, the mechanical characteristics of the hard disk, or the amount of the current IVCM required for keeping the VCM motor speed under control.
Shown in FIG. 2 is a timing diagram illustrating how the power-on signal INONOFF and synchronization signal INSH are linked with the two output voltage signals Out_P and Out_M of the power stage 3 and with the current IVCM flowing throughout the VCM motor 1.
It can be seen on the diagram that the electromotive force Bemf is detected at the end of the cut-off time period of the power stage 3, when the differential voltage at the two outputs of the power stage exactly matches the electromotive force Bemf generated by the movement of the VCM motor 1.
Also, during the enable time period (Ton) of the power stage 3, the current IVCM through the VCM motor 1 shows a step-wise pattern, specifically at its trailing edge.
FIG. 3a shows a current profile as detected in a prior art application. It can be seen from FIG. 3a that during the enable time period (Ton) of the power stage 3, no control is performed on the profile of the current IVCM flowing throughout the VCM motor 1.
Unfortunately, the current profile generated by prior art systems during the enable time period (Ton) is a major cause of the acoustical noise which is inherent to the drive arrangement discussed above.
There is accordingly a need to provide a system for controlling the speed of a VCM motor, specifically during the ramp loading of hard disk heads, with suitable design and performance features to still permit driving in the discontinuous PWM mode, but with a significant reduction in the acoustical noise caused by the loading operation, thereby overcoming the limitations of current systems according to the prior art.